Sevvel P
Associate Professor
Department of Mechanical Engineering,
Magna College of Engineering, Magaral,
Chennai – 600 055, Tamilnadu
India
Jaiganesh V
Professor
Department of Mechanical Engineering,
S.A. Engineering College,
Veeraraghavapuram, Thiruverkadu,
Chennai– 600 077, Tamilnadu
India
Impact of Tool Profile on Mechanical
Properties of AZ31B Mg Alloy during
FSW Using Optimized Parameters
In this experimental investigation, an attempt was made to understand the
influence of the tool profile on the mechanical properties of the joints
fabricated during friction stir welding (FSW) of AZ31B Mg alloy flat
plates. When the specimens prepared from the successfully fabricated
joints were subjected to tensile tests as per prescribed International
Standards it was observed that employing a taper cylindrical pin profiled
tool resulted in sound quality lap joints with improved mechanical
properties. Moreover, the use of other tool profiles namely straight
cylindrical&straightsquare could not produce defect free joints. Further,
this investigation reveals us that the common FSW defects including low
distortion, lack of cavity, tunnel defects etc are found to be completely
eliminated while using taper cylindrical pin profiled tool.
Keywords: AZ31B Mg alloy, Friction stir welding, tool profile, mechanical
properties.
1.
INTRODUCTION
Magnesium (Mg) alloys, being one of the lightest
structural materials, are nowadays widely replacing
copper, aluminium and steel alloys in various and
structural applications and automotive industries [1].
This is mainly due to their excellent properties including
high strength in combination with a low density, greater
thermal conductivity, sound castability and high
machinability [2]. Welding defects & solidification
problems including hot cracking, porosity, partial
melting zones, segregation of alloys etc are found to be
present with the adoption of fusion welding techniques
for Mg alloys [3].Hence, additional care should be taken
while selecting the suitable welding process for
fabricating parts composed of magnesium alloys.
Friction Stir Welding (FSW), an innovative type of
solid state welding technique invented & developed by
The Welding Institute (TWI), seems to provide higher
potential for joining magnesium alloys [4]. The main
advantage of FSW process is that the welding takes
place without melting the base metal, thereby
eliminating the welding defects like porosity, cracks etc
in the welded joints. Hence, this FSW is found to be
more suitable for welding magnesium alloys when
compared with the conventional resistance spot welding
techniques [5].
2.
LITERATURE SURVEY
Many experimental research works and investigations
have been conducted and carried out on different
Received: May 2015, Accepted: October 2015
Correspondence to: P. Sevvel
Department of Mechanical Engineering, Magna College
of Engineering, Magaral, Tamilnadu, India
E-mail: [email protected]
doi:10.5937/fmet1601043J
© Faculty of Mechanical Engineering, Belgrade. Allrights reserved
alloys of magnesium during their friction stir welding
under various conditions and process parameters [610]. Woo et al. [6] made a detailed investigation on the
AZ31B magnesium alloys and analyzed the spatial
variations being produced in their chemical
composition, hardness, microstructure, residual and
texture stresses during their friction stir processing
technique. They found that the friction stir processing
produced negligible amount of variations in the
chemical composition and hardness values of the
AZ31B magnesium alloys in their weld zone and the
size of the grains in the nugget zone have become finer
when compared with that of the grains present in the
base material. At the same time, they were able to find
major variations in the yield strength and texture
stresses in the friction stir processed zone and they
developed a numerical relationship between the
decrease in the yield strength values and stress
variations present in the stir zone.
Venkateswarlu et al. [8] studied the impact of the tool
designed with different shoulder diameters and their
ratio of overlapping during the friction stir welding of
AZ31 Mg alloy. Their work analyzed the effect of these
parameters on the formability of the material, its
mechanical properties including yield strength,
percentage of elongation and tensile strength. They were
able to produce joints containing fine refined grains
with dynamic recrystallization capabilities in the joint
area. They also proved that the use of the tool with
18mm diameter shoulder & 1.0 overlapping ratio was
able to produce joints with improved formability values
and fine homogeneous grain structure with grain size of
6.23 µm.
Dhanapal et al. [9] made an experimental attempt for
arriving at a mathematical relationship using response
surface technique to find out the rate of corrosion of the
AZ61A magnesium alloy joints fabricated using friction
stir welding. They made this investigation because of the
FME Transactions (2016) 44, 43-49 43
reason that the magnesium alloys possess a high affinity
for chemical agents which are oxidizing in nature and for
oxygen. This inherent nature of the magnesium alloys
with low levels of resistance towards corrosion plays an
important role in hindering these alloys for use in a
variety of industrial applications. The number of
experiments & conditions were decided using a three
factorial composite design. They successfully developed
a relationship which can be used to find out the rate of
corrosion associated with the friction stir welded joints of
AZ61A Mg alloys at a confidence level of 95%.
Dietzel et al [10] utilized friction stir welding
technique for joining AM50 magnesium alloy plates and
made an investigation on the observations in the
microstructures evolved during the process and the
tensile strength of the resulting joints. They were
successfully able to produce joints with 86.2% of the
tensile strength and 94 % of the yield strength of the
base material respectively. They also revealed that there
was a increase in the hardness value of the joints and the
welded joints exhibited fractures with a quasi-cleavage
feature unlike their parent metals with ductile fracture
nature. Moreover, with the help of the optical
microscopy images, they exhibited the presence of the
fine refined grain structures in the weld zone which was
very much smaller when compared with that of the
grain size present in the base metal.
In this paper, AZ31B magnesium alloy was taken
as the base metal and a detailed investigation was
carried to experimentally measure the role of three
different tool profiles namely straight square, straight
cylindrical & taper cylindrical tool pin profiles on
various mechanical properties including tensile
strength, yield strength and percentage of elongation
during the friction stir welding of this magnesium
alloy flat plates.
3.
EXPERIMENTAL WORK
Magnesium alloy is taken as the parent material for this
experimental investigation. The required thickness of
5mm of the AZ31B magnesium alloy was obtained by
machining the rolled plates to the required dimension.
The rolled plates are then cut to the required length &
width (150 X 50 mm) using power hacksaw followed by
the milling process, and Figure 1 shows the photograph
of the AZ31B Mg alloy specimens kept ready for joining
process using the friction stir welding technique. The
chemical compositions and mechanical properties of
AZ31B wrought magnesium alloy are listed in Table 1.
The AZ31B Mg alloy flat plates were firmly secured in
their position with the help of mechanical clamps and the
tool is held rigidly in its position as shown in the Figure 2.
The Friction Stir Welding (FSW) of AZ31B
magnesium alloys was performed using a semi
automatic FSW machine with a table size of 810 X 400
mm and a spindle speed ranging from 20 – 2000 rpm
with a feed range of 0.25 to 500 mm/min. This machine
has a motor power of 5 kW which can provide a 3 ton
mechanical linear axial force in the vertical direction.
Additionally, the machine has a 510 mm travel distance
along the X axis, 400 mm along the Y axis and 400 mm
with respect to the Z axis.
44 ▪ VOL. 44, No 1, 2016
Figure 1: Photographic View of the AZ31B Magnesium
Alloy flat plates of dimensions 150 X 50 X 5mm before the
friction stir welding process
Figure 2: Initial Joint Configuration arrangement of AZ31B
Mg Alloy flat plates in the Fixture along with the taper
cylindrical pin profiled HSS tool
The Friction Stir Welding (FSW) of AZ31B
magnesium alloys was performed using a semi
automatic FSW machine with a table size of 810 X 400
mm and a spindle speed ranging from 20 – 2000 rpm
with a feed range of 0.25 to 500 mm/min. This machine
has a motor power of 5 kW which can provide a 3 ton
mechanical linear axial force in the vertical direction.
Additionally, the machine has a 510 mm travel distance
along the X axis, 400 mm along the Y axis and 400 mm
with respect to the Z axis.
3.1 Tool material selection
In FSW, the material of the tool is one of the most
important key parameters in determining the quality of
the welded joints being produced. The selection of the
tool material is dependent on the material of the parent
metal to be welded. Moreover, selection of suitable tool
materials provides the opportunity of successfully
carrying out the FSW process at varying combinations
of rotational speeds and feed rates [11].
Non-consumable tool fabricated of M35 grade High
Speed Steel (HSS) is taken as the tool material in this
paper for FSW of AZ31B magnesium alloy flat plates.
The reason for choosing HSS as the tool material among
the different variety of available tool materials like High
Carbon High Chromium steel, Tool steel, carbide and
carbon boron nitride is its ease of availability, high hot
hardness, low cost, greater strengthand longer life time
[12]. The heat treatment details and chemical
composition of this HSS tool used in this paper are
described in Table 2.
FME Transactions
Table 1. Chemical composition & mechanical properties of AZ31B Mg alloy
Composition wt %
Mechanical Properties
Alloy
AZ31B
Mg alloy
Al
Zn
Mn
Cu
Si
Fe
Ni
Mg
2.5 – 3.5
0.6 – 1.4
0.2 – 1.0
0.05
0.10
0.005
0.005
balance
Tensile
strength
262
MPa
Yield
Strength
Elongation
percentage
179 MPa
8%
Table 2: Chemical composition & heat treatment details of M35 grade HSS Tool used in this paper
Composition wt %
Tool
Material
M35 grade
HSS
Heat Treatment Details
C
Si
Mn
Cr
Mo
V
W
Co
Annealing
(Slow
cooling)
Quenchent
(Hot Bath)
Tempering
(Air cooling)
0.92
0.35
0.3
4.1
5
1.9
6.4
4.8
800 – 840
oil
550 – 570
Quenched
& (hrc)
tempered
61
Design of tool pin profile
The tool pin profile and its geometry are found to play a
significant role in determining the generation of heat and
plastic flow during FSW. Moreover, the uniformity of
the welded joint produced and the soundness of the good
quality of the produced weld are directly dependant on
the tool pin profiles and their geometry [13].
The shoulder generally performs the function of
generating larger amount of heat and it alsoperforms
the function of preventing the plasticized material from
escaping the surface of the work piece. The pin profile
has been found to have a major input on the flow of
materials in the stir zone during the FSW process [14].
The three different tool pin geometries used in this
paper are straight cylindrical, straight square and taper
cylindrical. Figure 3 shows the photographic view of
these three different pin profiles.
Figure 3: Photographic view of HSS tools with different
pin profiles
The specifications and dimensions of the three
different tool pin profiles used in this investigation are
summarized in Table 3.
All the three tool different pin profile geometries
are obtained with the help of a CNC Turning centre
and optical profile type of grinding machine.The tools
manufactured using these machines are then subjected
to various suitable hardening processes to obtain a
hardness of 60–62 HRC. All the three tools with
different pin profile geometries have an outside
shoulder diameter of 20mm with a length of 50mm and
an inner shoulder diameter of 15mm with a length of
10mm. All the tools have a D/d ratio of 3.
FME Transactions
Table 3: Specifications and dimensions of the different
tool pin profiles used in this paper
Tool specifications & dimensions
Shape of
Pin
Minor
the tool pin Inner
Major Pin
Shoulder
Length
Pin
profile
Diameter
Diameter
L,
Diameter
(mm)
D (mm) (mm)
d, (mm)
Straight
12
4.75
4
4
Cylindrical
Straight
12
4.75
4
4
square
Taper
12
4.75
7
4
Cylindrical
D/d
ratio
of the
tool
3
3
3
Optimized welding parameters and fabrication
Since this paper principally investigates the impact of
the tool profile on the mechanical properties of the
friction stir welded AZ31B Mg alloy flat plates, the
other welding parameters, such as tool rotational
speed, welding speed and the axial force were set to
the already proven optimized values during the
friction stir welding of various grades of alloys like
aluminium & copper [15-17]. Friction stir welding
was successfully carried out on a number of AZ31B
Magnesium alloy flat plates using the three different
tool pin profiles at the optimized process parameters
which are listed in table 4 below.
Table 4: Optimized process parameters used during FSW
of AZ31B Mg alloy flat plates in this investigation
Process parameters
Tool rotational speed (rpm)
Feed rate (mm/min)
Axial force (kN)
Optimized values of the
process parameters
750
0.5
3
Friction stir welding is carried out in such a way that
the welding direction is normal to the rolling direction of
the specimen plates. Further, it is ensured that necessary
care was taken in order to avoid distortion during the
joining process and the joints were fabricated using the
single pass welding technique.
VOL. 44, No 1, 2016 ▪ 45
4.
AZ31B magnesium specimens using three different pin
profiles are clearly displayed in the Figure 5.
Of the various joints fabricated during this process,
the joint fabricated using a taper cylindrical pin profiled
tool was found to be free from welding defects and had
been observed as a defect free joint, as shown in the
Figure 5c. This may be one of the reasons for obtaining a
higher value of tensile strength with the same joint.
The joints fabricated with the straight cylindrical were
observed to be present with minimum amount of tunnel
defect. At the same time, the use of straight square pin
profiled tool was found to produce joints in which the
larger tunnel defects were present. From the macroscopic
observations of the welded specimens shown in Figure 5,
it had been experimentally found that the use of the taper
cylindrical pin profiled tool under optimized parameters
of 750 rotational speed, 0.5mm/min feed rate and 3kN
axial force values was found to produce sound and better
weld quality defect free joints with negligible defects and
even a completely defect free joint when compared with
other two pin profiled tools.
RESULTS AND DISCUSSIONS
Appearance of joints
The photographic view of sample of the joints
successfully fabricated during the friction stir welding
of the AZ31B magnesium alloy flat platesusing the
M35 grade HSS tool with three different pin profile
geometries namely straight cylindrical, straight square
and taper cylindrical under optimized process
parameters are illustrated in Figure 4.
Macrostructure of the FSW joints
The top surface of the majority of the welded joints
seemed to be free from visible defects. However, the
cross sections of some of the welded specimens are
found to be present with tunnel defects in the various
sections of the welded zone. These defects were
observed when the welded specimens were analysed
under the low magnification (10X) using the optical
microscope and the macrostructure of the fabricated
Figure 4: Photographic View of the sample of the AZ31B magnesium alloy joints successfully fabricated by the friction stir
welding process using three different tool pin profiles at optimized process parameters
(a)
(b)
(c)
Figure 5: Photographic Image of the macrostructure of the fabricated AZ31B magnesium specimens by three different pin
profiles namely (a) Straight square (b) straight cylindrical (c) taper cylindrical under optimized process parameters
46 ▪ VOL. 44, No 1, 2016
FME Transactions
Mechanical properties
In order to investigate and characterize the mechanical
properties of the friction stir welded specimens using
three different tool pin profiles, a series of tensile shear
tests were conducted as per the International standards.
Procedures prescribed by the American Society for
Testing of Materials (ASTM: B557M–10) standard
guidelines were adopted during the preparation of the
specimens for carrying out the tensile tests. Photograph
of the tensile test specimens prepared by the above
mentioned standard are illustrated in Figure 6.
Each of two to three specimens produced using
three different pin profiles were subjected to tensile
testing and the stress strain graph generated during this
tensile testing for the specimen welded using different
pin profiles are shown in the Figure 9. As depicted in
the Figure 9 (c), of the various welded joints, the joints
fabricated using taper cylindrical pin profile was found
to yield better tensile properties. The ultimate tensile
strength exhibited by this joint specimen is 168 MPa
which is approximately 64.12% of the tensile strength
of the AZ31B magnesium alloy (262 MPa). Moreover,
the yield strength exhibited by this joint is 98 MPa
which is approximately 54.7% than that of the parent
metal (179 MPa). The related percentage of elongation
for this joint is 5.45%.
Figure 6: Tensile specimen prepared as per ASTM:
B557M–10 standard guidelines
The lengths of the tensile test specimens were cut
along the processed direction. The shape of the tensile
test specimens were obtained by slicing the single pass
friction stir welded pieces using power hacksaw
machine. The photographic view of the prepared tensile
specimens as per the above mentioned processes can be
seen in the Figure 7 and these tensile test specimens
were used to evaluate the tensile strength, yield
strength and % elongation of the friction stir welded
AZ31B Mg alloy plates.
Figure 7: Photographic View of the tensile test specimen
prepared as per the ASTM: B557M–10 standard guidelines
before tensile testing
Figure 9: Stress strain graph generated during the tensile
testing for the specimen welded using (a) Straight square
(b) straight cylindrical (c) taper cylindrical pin profiles
Figure 8: Photographic View of the sample of the tensile
test specimen after tensile testing
After machining, the prepared tensile specimens
were inspected and loaded at the rate of 1kN/min as per
the ASTM guidelines and Figure 8 shows the
photographic view of the tensile test specimens after
the completion of the tensile testing using the universal
testing machine.
FME Transactions
5.
CONCLUSIONS & FUTURE SCOPE
In this paper, a detailed investigation had been
carried out to find out the changes being produced in
the macro structural characterization and mechanical
properties of the AZ31B Mg alloy by using three
different tool pin profiles at the optimized process
parameter values including tool rotational speed of 750
rpm, 0.5mm/min feed rate and axial force of 3 kN
VOL. 44, No 1, 2016 ▪ 47
during the FSW. The following conclusions have been
experimentally derived:
1) It had been experimentally proved that for joining
of AZ31B Mg alloy flat plates of 5mm thickness
using the FSW technique at the optimized process
parameters, taper cylindrical pin profiled is
preferable compared with the other tool pin
profiles and this taper cylindrical pin profiled tool
was found to exhibit improved & better
mechanical properties when compared with the
other two pin profiles under FSW parameters of
optimized values.
2) The tensile strength value produced using the taper
cylindrical pin profile is about 64.12 % of the
strength of the base metal. Likewise, the related
yield strength value is nearly 54.5% of its base
metal and the percentage of elongation being
5.45%.
Additionally, the paper also provides a wide scope
for carrying out future experimental and numerical
investigations on enhancing and improving the micro
structural characterization of the AZ31B Mg alloy
joints using the taper cylindrical pin profiled tool
during the FSW process.
ACKNOWLEDGMENT
The authors gratefully thank the Management and
Mechanical Engg Dept., S.A Engineering College,
Chennai, India for providing the welding equipment
facilities. The authors wish to express thanks to All
India Council for Technical Education (AICTE) Govt.
of INDIA, funded project (Grant No. 8023/RID/RPS/
037/2011-12) for sponsoring FSW machine.
REFERENCES
[1] Cao, X. and Jahazi M.: Effect of tool rotational speed
and probe length on lap joint quality of a friction stir
welded Mg alloy, Mater Des Vol. 32, pp. 01 – 11,
2011.
[2] Mordike, B.L. and Ebert, T.: Magnesium:
properties – applications – potential, Materials
Science Engineering A, pp. 37 – 45, 2000.
[3] Avedeelan, M.M. and Baker,H.: Magnesium and
Magnesium Alloys, ASM Special Handbook,
ASM International, pp. 106 – 118, 1999.
[4] Sevvel, P. and Jaiganesh, V.: An detailed
examination on the future prospects of friction stir
welding – a green technology, Proceedings of the
International
Conference on Advances In
Industrial Engineering Applications – ICAIEA
2014, January 06 – 08, pp. 62 – 63, 2014.
[5] Nandan, R., DebRoy, T. and Bhadeshia,H.K.D.H.:
Recent trends in friction stir welding process,
weldment structure and properties, Prog. Mater.
Sci., pp. 980–1023, 2008.
[6] Woo, W., Choo, H., Prime, M.B., Feng, Z. and
Clausen,B.: Microstructure, texture and residual
stress in a friction-stir-processed AZ31B
magnesium alloy, Acta Mater., Vol. 56, No. 8, pp.
1701–1711, May 2008.
48 ▪ VOL. 44, No 1, 2016
[7] Ramesh Babu,S., Senthil Kumar,V. S.,
Karunamoorthy, L. and Madhusudhan Reddy,G.:
Investigation on the effect of friction stir
processing on the superplastic forming of AZ31B
alloy, Mater. Des., Vol. 53, pp. 338–348, Jan.
2014.
[8] Zhao,Y.H., Lin,S.B., Qu,F. X. and Wu,L.:
Influence of pin geometry on materialflow in
friction stir welding process, Materials Science and
Technology, vol. 22(1), pp. 45–50, 2006.
[9] Dhanapal,A., Boopathy,S.R. and Balasubramanian,
V.: Developing an empirical relationship to predict
the corrosion rate of friction stir welded AZ61A
magnesium alloy under salt fog environment,
Mater. Des., Vol. 32, no. 10, pp. 5066–5072, Dec.
2011
[10] Zeng,R., Dietzel,W., Zettler,R., Chen,J. and
Kainer,K. U.: Microstructure evolution and tensile
properties of friction-stir-welded AM50 magnesium
alloy, Trans. Nonferrous Met. Soc. China, Vol. 18,
pp. s76–s80, Dec. 2008.
[11] Sevvel, P. and Jaiganesh,V.: A detailed
investigation on the role of different Tool
Geometry in Friction Stir Welding of various
Metals & their Alloys, Proceedings of the
International
Colloquium
on
Materials,
Manufacturing & Metrology – ICMMM, August 89, Pp 103 – 107, 2014.
[12] Ganesa Balamurugan, K. and Mahadevan,K.:
Investigation on the changes effected by tool
profile on mechanical and tribological properties
of friction stir processed AZ31B magnesium alloy,
J. Manuf. Process., Vol. 15, No. 4, pp. 659–665,
2013.
[13] Tozaki,Y., Uematsu,Y. and Tokaji,K.: A newly
developed tool without probe for friction stir spot
welding and its performance, J. Mater. Process.
Technol., Vol. 210, pp.844–851, 2010.
[14] Sevvel, P. and Jaiganesh,V.: Experimental
Investigation on the impact of the Tool Material
& geometry in joining of Al 63400 Alloy using
Friction Stir Welding Process, Applied
Mechanics and Materials Vol. 592-594, pp. 312315, 2014.
[15] Lombord,H., Hattingh,D.G., Steuwer,A. and
James,M.N.: Optimizing FSW process parameters
to minimize defects and maximizing fatigue life in
5083 – H321 aluminium alloy, J. Eng Fract Mech,
pp. 341 – 354, 2008.
[16] Sevvel, P. and Jaiganesh,V.: "Improving the
mechanical properties of friction stir welded
AZ31B magnesium alloy flat plates through
axial force investigation", vol. 591, pp. 11–14,
2014.
[17] Sun, Y.F. and Fujii,H.: Investigation of the
welding parameter dependent microstructure and
mechanical properties of friction stir welded pure
copper, Mater. Sci. Eng. A, Vol. 527, no. 26, pp.
6879–6886, 2010.
FME Transactions
УТИЦАЈ ПРОФИЛА АЛАТА НА МЕХАНИЧКЕ
КАРАКТЕРИСТИКЕ МАГНЕЗИЈУМОВЕ
ЛЕГУРЕ AZ31B КОД ЗАВАРИВАЊА ТРЕЊЕМ
СА МЕШАЊЕМ ПРИМЕНОМ
ОПТИМИЗИРАНИХ ПАРАМЕТАРА
Sevvel P., Jaiganesh V.
Овим експерименталним истраживањем покушали
смо да разумемо колико профил алата утиче на
механичке карактеристике склопова од равних
лимова произведених од магнезијумове легуре
AZ31B и добијених заваривањем трењем са
FME Transactions
мешањем. Када су узорци успешно припремљени
од произведених склопова изложени су испитивању
затезањем према међународним стандардима,
уочили смо да се коришћењем зашиљених
коничних чивија добија квалитетнији лим са
побољшаним механичким својствима. Осим тога,
применом других профила алата, тј. чивија са
коничним и квадратним профилом, добили смо
спојеве са дефектима. Ово истраживање је показало
да уобичајени дефекти код заваривања трењем са
мешањем, као што је мала дисторзија, одсуство
шупљина у потпуности се отклањају коришћењем
алата са профилом зашиљене коничне чивије.
VOL. 44, No 1, 2016 ▪ 49